There are many applications in which there is a need to measure changes in the relative positioning between components. In many such applications, the components are readily accessible, and measurements can be made directly. However, in other situations, one or more of the components may not be readily accessible, and thus some mechanism for indirect measurement must be used. One example situation which presents specific difficulties is in tools that may be used in wells or other subsurface locations. For example, as some configurations of a downhole tool are operated within a well bore, various pistons and other moving parts may move relative other portions of the downhole tool. An example of one such moving part might be a piston within a chamber of the downhole tool, such as might be used, for example, for sampling fluids from the formation or well annulus. In this example, measurement of displacement of the piston relative to a surface of the chamber within which it is housed may provide useful information regarding the volume of that chamber, and therefore of the fluid therein.
One difficulty in this example operating environment is that the system needs to be operable, and generally stable, under elevated temperatures, and must be sufficiently rugged to withstand the downhole environment. These extreme conditions in a typical well bore make it difficult to provide tools that can properly determine relative positions of moving members downhole.